The attainment of aesthetics in synthetic tooth filling/covering materials demands close control over many factors. The ideal prosthetic material blends well with the restored tooth as well as with adjacent teeth under widely-varying conditions of illumination, including natural and artificial light, and even under ultraviolet or "black" light. Obviously, the color of the material and the ability of the material to transmit a certain fraction of the light which impinges thereon are important. Excellent aesthetics also requires that the material exhibit the capability to disperse and scatter the light through internal reflections. That capability produces a certain depth to the translucency and color which can mimic such natural materials as tooth enamel and ivory.
Glass-ceramics have been used commercially as synthetic tooth filling/covering materials and in dental prostheses [U.S. Pat. No. 4,431,420 (Adair) and U.S. Pat. No. 4,652,312 (Grossman et al.)], because of the many fine scattering centers produced by internal nucleation and controlled growth of crystals in those highly crystalline materials. Fabrication of those glass-ceramic materials into the precise shapes demanded for tooth repair and replacement has utilized several techniques, the two most commonly being practiced involving casting via the classic lost wax process and the rather recent CAD/CAM technique. [U.S. Pat. No. 4,575,805 (Moermann et al.) and U.S. Pat. No. 4,663,720 (Duret et al.)].
U.S. Pat. No. 3,839,055 (Grossman) describes a family of glass-ceramic materials identified as tetrasilicic fluormica glass-ceramics which are characterized by good strength and translucency, thereby rendering them especially well suited for the fabrication of dental constructs. European Patent No. 0083828 (Grossman) delineates a preferred range of tetrasilicic fluormica base compositions for dental applications. Those preferred base compositions consist essentially, in weight percent as calculated from the batch, of 45-70% SiO.sub.2, 8-20% MgO, 8-15% MgF.sub.2, 5-20% K.sub.2 O, 0.05-2% Al.sub.2 O.sub.3, 0.5-7% ZrO.sub.2, 1-9% total ZrO.sub.2 +Al.sub.2 O.sub.3, 0-7% TiO.sub.2, and 0-10% conventional glass colorants. The patent mentions the optional inclusion of Group II metal oxides and oxides of the metallic transition elements. U.S. Pat. No. 4,652,312, supra, describes a further improved range of tetrasilicic fluormica base compositions for dental applications. Those compositions consist essentially, in weight percent on the oxide basis, except for fluorine which is expressed on an elemental basis, of 45-70% SiO.sub.2, 13-30% MgO, 5-20% K.sub.2 O, 4-9% F, 0-2% Al.sub.2 O.sub.3, 0-7% ZrO.sub.2, 1-4% BaO, and 0-5% SrO. The most preferred compositions consist essentially, in weight percent, of 55-65% SiO.sub.2, 14-19% MgO, 8-18% K.sub.2 O, 0.05-2% Al.sub.2 O.sub.3, 0.5-7% ZrO.sub.2, 4-9% F, and 1-4% BaO.
In conventional dental ceramics, such as dental porcelain, where it is desired to produce a given color in the fired material, a ceramic pigment or combination of several pigments is incorporated into the composition. The crystals of the pigments remain sufficiently intact during the firing process to impart the desired color in the finished product. Ceramic pigments consist mostly of refractory crystals containing color centers frequently produced by the ions of such transition metals as Co, Cr, Fe, Mn, Ni, and V. The actual color provided by those pigments, however, is governed to a large extent by the crystalline structure of the host compound into which the transition metal ions are incorporated.
To color a glass or a glass-ceramic article, coloring ions such as those in the transition metal series are customarily added to the precursor batch as oxides or carbonates. Those compounds are dissolved in the glass during melting and the color center produced depends upon the individual ion(s) and the ligand field surrounding the ion(s), the latter being determined by the glass or amorphous structure. This method of coloring is more analogous to solution chemistry and is distinct from the prior method conventionally utilized in ceramic technology.
In the field of glass-ceramics it is quite common for the color produced in the precursor or parent glass to be different from the color developed in the crystallized glass-ceramic as a result of heat treating the parent glass. This change is brought about through the alteration in the amorphous structure which occurs to the composition of the residual glass as the components comprising the crystals are removed therefrom. There may also be some incorporation of the coloring ions into the crystals of the glass-ceramic which would likewise alter the ligand field surrounding the coloring ions.
In order to produce acceptable colors for use in dental glass-ceramics, colorants are needed which produce yellow to yellow-red or tan in the crystallized product. Unfortunately, no single transition metal or rare earth metal ion has been identified which yields a clear yellow effect with the exception of uranium. Governmental regulations regarding radioactivity restrict the use of uranium. However, a method for combining certain pairs of ions to gain an interactive effect leading to yellow colors has been described by W. A. Weyl in "Coloured Glasses", Dawson's of Pall Mall, London, England, 1959.
For example, Weyl reports the use of CeO.sub.2 which, by itself produces no color in glass, but which, in combination with TiO.sub.2 imparts strong yellow colors. That yellow color can be modified by incorporating the rare earth metal oxide Er.sub.2 O.sub.3 therein, and other color modifiers may be utilized to vary the shade produced. Whereas the combination of CeO.sub.2 and TiO.sub.2 with, perhaps, other modifying oxides works well in visible light, fluorescence under long wave ultraviolet illumination does not take place because the presence of TiO.sub.2 strongly absorbs ultraviolet radiation.
Therefore, the primary objective of the present invention was to devise glass-ceramic compositions which would exhibit a range of yellow colorations in the visible portion of the radiation spectrum, coupled with the ability to fluoresce under ultraviolet illumination. Such glass-ceramic materials would be especially useful as dental prosthetic materials in that their colors can be modified to blend well with a restored tooth as well as with adjacent teeth under various conditions of illumination, including visible and ultraviolet light.